1. Field of Invention
This invention relates generally to optical storage systems and, more particularly, to the detection apparatus associated with a write-once system by which the change in reflectivity of an irradiated region of a storage region, and hence the binary state represented by the region, is identified.
2. Description of the Related Art
In the write-once optical storage system, noise can be introduced into the system by several mechanisms. For example, the radiation detectors can introduce shot noise and thermal (Johnson) noise into the signal. The effect of the shot noise and the thermal noise can not be reduced by differential detection systems. However, one of the largest sources of noise are the instabilities in the laser radiation unit. In certain applications in which a differential detection system is employed, a reduction in noise can automatically be achieved.
Referring to FIG. 1, a write once-optical information storage system is illustrated. A radiation beam is generated by means of a laser diode 11. The radiation beam is collimated by collimating lens 12 and applied to beam splitter 13. The portion of the radiation beam with a polarization determined by the characteristics of the beam splitter 13 is transmitted by the beam splitter 13 and applied to quarter wave plate 14. The portion of the radiation beam which is not transmitted by the beam splitter 13 is reflected thereby and leaves the optical path of the storage system. The portion of the radiation beam transmitted by the quarter wave plate 14 is applied to objective lens 15. The objective lens focuses the radiation beam through a protective layer 5B and onto an information bearing surface 5A of the storage medium 5. The radiation beam is reflected from the storage medium surface 5A and is recollimated by objective lens 15 The recollimated radiation beam is transmitted through the quarter-wave plate 14, and applied to polarization beam splitter 13. The portion of the reflected radiation beam which has been rotated by interaction with the storage medium and the quarter wave plate will have a linearly polarized radiation component rotated by an angle of 90.degree. from the polarized radiation component transmitted by the beam splitter 13. The rotated radiation component will be reflected by the beam splitter 13 and applied to radiation detector 16. The portion of the radiation beam which has not been rotated by interaction with the storage medium will be transmitted through the polarization beam splitter 13. This radiation component transmitted by the beam splitter will remain as part of the laser cavity radiation. The output signal from radiation detector 16 is applied to amplifier 17. The output signal from the amplifier can be processed to determine the information stored on the storage medium.
The operation of the write once information storage system can be understood in the following manner. The memory layer 5A of the storage medium is fabricated so that the reflectivity of selected regions of the storage medium can be altered with respect to the surrounding region. When information is stored on the memory layer, the reflectivity of the selected regions, establishes along a predetermined path or track the interpretation of selected regions as a sequence of logical signals. The quarter wave plate converts the linearly polarized radiation into a circularly polarized radiation beam. When the circularly polarized radiation is reflected from the storage layer, the differential absorption between the two orientation states of the storage layer will provide a detectable difference in the signal amplitude. Therefore, the reflectivity state of the currently illuminated portion of the storage layer can be determined from the output signal of amplifier 17. The determination of the reflectivity of the illuminated selected region permits the correlation of the illuminated region of the storage layer with a logic state and, consequently, the retrieval of information stored on the storage layer.
As will be clear to those skilled in the art, the differential absorption can be relatively small. Therefore, the signal or modulated components of the radiation beam can be relatively small and a carrier signal to noise ratio (CNR) can be small. One of the principal contributions to the noise is the result of instabilities in the laser unit. In order to compensate for laser unit instabilities, one technique has been to extract a portion of the laser radiation from the radiation source and compensate for noise in the signal-bearing radiation beam by combining the extracted radiation portion with the signal-bearing radiation source. An example of this compensation is found in U.S. Pat. No. 4,896,222 issued in the name of Fukai. In U.S. Pat. No. 4,150,402 issued in the name of Tietze et al., a portion of the radiation beam is used to determine a general level of the laser radiation, thereby permitting a calibration for a signal-bearing or modulated beam. In U.S. Pat. No. 5,105,413 issued in the name of Bakx, a technique for dividing a reflected radiation from a recording media to remove the effects of write modulation is disclosed. However, none of the references disclose a technique for reducing the laser noise in an optical recording head.